Convergent charged particle beam apparatus and inspection method using same

ABSTRACT

A convergent charged particle beam apparatus includes an electron beam system which emits a converged electron beam, a vacuum chamber which is connected to the electron beam system, and a stage which mounts a specimen and moves at least in one direction inside of the vacuum chamber. An electron beam image observation unit observes an electron beam image of a surface of the specimen. A height detector optically detects a height of the specimen mounted on the stage by illuminating the surface of said specimen with light and by detecting reflected light of the illumination reflected from the surface of the specimen, and a controller controls a focus position of the electron beam in accordance with an output from the height detector while the stage is moving at least in one direction.

CROSS REFERENCE TO RELATED APPLICATION

[0001] This application a continuation application of U.S. applicationSer. No. 10/426,702, filed May 1, 2003, which is a continuation of U.S.application Ser. No. 10/012,400, filed Dec. 12, 2001, now U.S. Pat. No.6,559,459, which is a continuation of U.S. application Ser.No.09/258,461, filed Feb. 26, 1999, now U.S. Pat. No. 6,335,532, whichis a continuation-in-part application of U.S. application Ser. No.09/132,220, filed Aug. 11, 1998, by some of the inventors herein, nowU.S. Pat. No. 6,107,637, the subject matter of the aforementionedapplications being incorporated by reference herein.

BACKGROUND OF THE INVENTION

[0002] The present invention relates to a convergent charged particlebeam apparatus using a charged particle beam such as an electron beam orion beam for microstructure fabrication or observation and an inspectionmethod using the same, and more particularly to an automatic focusingsystem and arrangement in the convergent charged particle beamapparatus.

[0003] As an example of an apparatus using a charged particle beam,there is an automatic inspection system intended for inspecting andmeasuring a microcircuit pattern formed on a substrate such as asemiconductor wafer. In defect inspection of a microcircuit patternformed on a semiconductor wafer or the like, the microcircuit patternunder test is compared with a verified non-defective pattern or anycorresponding pattern on the wafer under inspection. A variety ofoptical micrograph imaging instruments have been put to practical usefor this purpose, and also electron micrograph imaging has foundprogressive applications to defect inspection by pattern imagecomparison. In a scanning electron microscope instrument which isspecifically designed for critical-dimension measurement of line widthsand hole diameters on microcircuit patterns used for setting andmonitoring process conditions of semiconductor device fabricationequipment, automatic critical-dimension measurement is implementedthrough use of image processing.

[0004] In comparison inspection where electron beam images ofcorresponding microcircuit patterns are compared for detecting apossible defect or in critical-dimension measurement where electron beamimages are processed for measuring such dimensions as pattern linewidths, reliability of results of inspection or measurement largelydepends on the quality of electron beam images. Deterioration inelectron beam image quality occurs due to image distortion caused bydeflection or aberration in electron optics, decreased resolution causedby defocusing, etc., resulting in degradation of performance incomparison inspection or critical-dimension measurement.

[0005] In a situation where a specimen surface is not uniform in height,if inspection is conducted on the entire surface area under the samecondition, an electron beam image varies with each region inspected asexemplified in FIGS. 1(a)-1(d), wherein FIG. 1(a) shows a wafer withdifferent regions A-C, FIG. 1(b) shows an in-focus image of region A andFIGS. 1(c) and 1(d) show defocused images of regions B and C,respectively. In inspection by comparison between the in-focus image ofFIG. 1(b) and the defocused image FIG. 1(c) or FIG. 1(d), it isimpossible to attain correct results. Further, since these imagesprovide variation in pattern dimensions and results of edge detection onthem are unstable, pattern line widths and hole diameters cannot bemeasured accurately. Conventionally, image focusing on an electronmicroscope is performed by adjusting a control current to an objectivelens thereof while observing an electron beam image. This procedurerequires a substantial amount of time and involves repetitive scanningon a surface of a specimen, which may cause a possible problem ofspecimen damage.

[0006] In Japanese Non-examined Patent Publication No. 258703/1993,there is disclosed a method intended for circumventing theabovementioned disadvantages, wherein an optimum control current to anobjective lens for each surface height of a specimen is pre-measured atsome points on the specimen and then, at the time of inspection, focusadjustment at each point is made by interpolation of pre-measured data.However, this method is also disadvantageous in that a considerableamount of time is required for measuring an optimum objective lenscontrol current before inspection and each specimen surface height mayvary during inspection depending on wafer holding conditions.

[0007] A focus adjustment method for a scanning electron microscopeusing an optical height detecting arrangement is found in JapaneseNon-examined Patent Publication No. 254649/1988. However, since anoptical element for height detection is disposed in a vacuum system, itis rather difficult to perform optical axis alignment.

[0008] In microstructure fabricating equipment using a convergentcharged particle beam, focus adjustment of the charged particle beam hasa significant effect on fabrication accuracy, i.e., focus adjustment isof extreme importance as in instruments designed for observation.Examples of microstructure fabricating equipment include an electronbeam exposure system for forming semiconductor circuit patterns, afocused ion beam (FIB) system for repairing circuit patterns, etc.

[0009] In a scanning electron microscope, a method of measuring anoptimum control current to an objective lens thereof through electronbeam imaging necessitates attaining a plurality of electron beam imagesfor detecting a focal point, thus requiring a considerable amount oftime for focus adjustment. That is, such a method is not suitable forfocusing in a short time. Further, in an application of automaticinspection or critical-dimension measurement over a wide range, focusadjustment at every point using the abovementioned method is notpracticable, and it is therefore required to perform pre-measurement atsome points before inspection and then estimate a height at each pointthrough interpolation, for instance. FIG. 2 shows an overview of anelectron-beam automatic semiconductor device inspection system to whichthe present invention is directed. In such an automatic inspectionsystem, a specimen wafer under inspection is moved by means of stageswith respect to an electron optical system thereof for carrying outwide-range inspection.

[0010] A semiconductor wafer to be inspected in a fabrication processmay deform due to heat treatment or other processing, and a degree ofdeformation will be on the order of some hundreds of micrometers in theworst case. However, it is extremely difficult to hold the specimenwafer stably without causing interference with electron optics in avacuum specimen chamber, and also it is impossible to adjust specimenleveling as in an optical inspection system using vacuum chucking.

[0011] Further, since a substantial amount of time is required forinspection, a specimen holding state may vary due toacceleration/deceleration in reciprocating stage movement, therebyresulting in a specimen surface height being different from apre-measured level.

[0012] For the reasons mentioned above, there is a rather high degree ofpossibility that a surface height of a specimen under inspection willvary unstably exceeding a focal depth of the electron optical system (adepth of focus is generally on the order of micrometers at amagnification of 100×, but that necessary for semiconductor deviceinspection depends on inspection performance requirements concerned).For focus adjustment using electron beam images, a plurality of electronimages must be attained at each point of interest with each stage beingstopped. It is impossible to conduct focus adjustment continuously whiledetecting a height at each point simultaneously with stage movement forthe specimen under inspection.

[0013] In an approach that focus adjustment using electron beam imagesis performed at some points on a specimen surface before the start ofinspection, an amount of time is required for calibration beforeinspection. This causes a significant decrease in throughput as a sizeof wafer becomes larger. Since there is a technological trend towardlarger-diameter wafers, a degree of wafer deformation such as bowing orwarping will tend to be larger, resulting in more stringent requirementsbeing imposed on automatic focusing functionality. Depending on thematerial of a specimen, exposure with an electron beam may alter anelectric charge state on specimen surface to cause an adverse effect onelectron beam images used for inspection.

[0014] In consideration of the above, it is difficult to ensuresatisfactory performance in long-period inspection on a scanningelectron microscope instrument using the conventional methods. Wherestable holding of a specimen is rather difficult, it is desirable tocarry out specimen surface height detection in a range of electronoptical observation immediately before images are attained duringinspection. Further, where inspection is conducted while each stage ismoved continuously, specimen surface height detection must also becarried out continuously at high speed without interrupting a flow ofinspection operation. For realizing continuous surface height detectionsimultaneously with inspection, it is required to detect a height ofeach inspection position or its vicinity at high speed.

[0015] However, if any element which affects an electric or magneticfield, e.g., an insulating or magnetic element, is disposed in thevicinity of an observation region, electron beam scanning is affectedadversely. It is therefore impracticable to mount a sensor in thevicinity of electron optics. Further, since the observation region islocated in the vacuum specimen chamber, measurement must be enabled in avacuum. For use in the vacuum specimen chamber, it is also desirable tomake easy adjustment and maintenance available. While there have beendescribed conditions as to an example of an electron-beam inspectionsystem, these conditions are also the same in a microstructureobservation/fabrication system using an ion beam or any other convergentcharged particle beam. Further, since there are the same conditions insuch systems that images of an aperture, mask, etc. are formed orprojected as well as in a system where a charged particle beam isconverged into a single point, it is apparent that the present inventionis applicable to charged particle beam systems comprising any chargedparticle beam optics for image formation/projection.

SUMMARY OF THE INVENTION

[0016] It is therefore an object of the present invention to provide anarrangement for detecting a surface height of an object item in anobservation/fabrication region of charged particle beam optics or in itsvicinity under vacuum at high speed without causing interference withthe charged particle beam optics.

[0017] According to the present invention, there is provided a highlyreliable system in which the object item can be observed/fabricated withits image being always in focus using surface height data thus detected.

[0018] According to an embodiment of the present invention, there isprovided a height detector capable of detecting a surface height of anobject item in an observation/fabrication region without causinginterference with charged particle beam optics simultaneously withobservation/fabrication and a system capable of carrying outobservation/fabrication using a charged particle beam image formed inthe charged particle beam optics in which focus adjustment can be madewith height data obtained through the height detector. For enablingspecimen height detection without causing interference with the chargedparticle beam optics, it is necessary to provide a height detector whichcan detect a surface height from a distant position apart from thecharged particle beam optics. Further, for preventing an adverse effecton charged particle beam scanning, a height detection method must bearranged so that influence on electric and magnetic fields in thevicinity of a detection position will not vary with time. Additionally,since a specimen chamber is evaluated, the height detector must beusable under vacuum.

[0019] According to one aspect of the present invention, there isprovided a height detector based on an optical height detection methodin which light is projected to a height detection position slantwise andreflected light from a specimen surface is measured for heightdetection.

[0020] In accordance with the present invention, a convergent chargedparticle beam system comprises an electron beam source, an electronoptical system unit for converging an electron beam emitted from theelectron beam source into focus, a vacuum chamber unit having the insidethereof evacuated, a stage unit arranged in the inside of the vacuumchamber means so as to mount a specimen under inspection thereon andmove the specimen along each plane, an electron beam image observationunit for observing an electron beam image of a surface of the specimenmounted on the stage unit in a manner that the electron beam convergedby the electron optical system means is scanned over the surface of thespecimen for irradiation and secondary charged particles produced fromthe specimen are detected, a height detecting unit for opticallydetecting a specimen surface height in a region irradiated with theelectron beam scanned by the electron optical system means, a controlunit for controlling a focal point of the electron beam converged by theelectron optical system unit and a heightwise relative position of thespecimen through use of resultant data detected by the height detectingunit, and a defect detecting unit for detecting a possible defect on thespecimen by processing electron beam image data of the surface of thespecimen observed by the electron beam image observation means in astate that the focal point of the electron beam and the heightwiserelative position of the specimen are controlled by the control unit.

[0021] Further, in accordance with the present invention, there isprovided an inspection method using a convergent charged particle beamsystem, comprising the steps of setting a specimen under inspection on amovable table inside a processing chamber, evacuating the processingchamber containing the specimen, scanning an electron beam emitted froman electron beam source while moving the movable table in the inside ofthe evacuated processing chamber to optically detect a height of ascanning-electron-beam-irradiated region on a surface of the specimen inan optical axis direction of the electron beam source, adjusting asurface height of the specimen in the optical axis direction accordingto resultant height data thus detected, scanning the electron beamemitted from the electron beam source for irradiation over the specimenthus adjusted in height while moving the movable table, detectingsecondary charged particles produced from the specimen irradiated withthe electron beam through scanning to attain a secondary chargedparticle image of the surface of the specimen, and inspecting thespecimen using the secondary charged particle image thus attained.

[0022] These and other objects, features and advantages of the presentinvention will become more apparent from the following description whentaken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] FIGS. 1(a)-1(d) show inspection of a wafer at different regionsand electron beam images of the different regions;

[0024]FIG. 2 is a schematic sectional view showing an exemplarystructure of an automatic inspection system according to the presentinvention;

[0025]FIG. 3 is a schematic sectional view of a height detection opticalsystem for illustrating a principle of height detection;

[0026]FIG. 4 is a graph showing variation in reflectance with respect toincidence angle on each material;

[0027]FIG. 5 is a schematic sectional view of a specimen chamber,showing an example of altered disposition of height detection opticalsystem parts;

[0028]FIG. 6 is a schematic sectional view of a specimen chamber,showing an arrangement in which the height detection optical systemparts are disposed outside the specimen chamber;

[0029]FIG. 7 is a schematic sectional view of a specimen chamber,showing an arrangement in which the height detection optical systemparts are disposed inside the specimen chamber;

[0030]FIG. 8 is a schematic sectional view of a specimen chamber,showing an arrangement in which optical path windows are formed along aplane of an external top wall of the specimen chamber;

[0031]FIG. 9 is a graph showing variation in reflectance with respect toincidence angle on glass BK7;

[0032]FIG. 10 is a schematic sectional view of a specimen chamber,showing an arrangement in which optical path windows are formedperpendicularly to an optical path on an external top wall of thespecimen chamber;

[0033]FIG. 11 is a schematic sectional view illustrating chromaticaberration due to a glass window;

[0034]FIG. 12 is a schematic sectional view illustrating an arrangementin which a glass plate is inserted for correction of chromaticaberration due to a glass window;

[0035]FIG. 13 is a schematic sectional view illustrating anotherarrangement in which a glass plate is inserted in a different manner forcorrection of chromatic aberration due to a glass window;

[0036] FIGS. 14(a) and (b) are schematic sectional views showing achange in optical path size on a flat-plate electrode according toincidence angle;

[0037]FIG. 15 is a schematic sectional view showing a shape of anentrance opening on the flat-plate electrode in case of a circularoptical aperture;

[0038]FIG. 16 is a schematic sectional view showing a shape of anentrance opening on the flat-plate electrode in case of an ellipticaloptical aperture;

[0039]FIG. 17 is a schematic sectional view showing an example of anwindow formed perpendicularly to an optical path on the flat-plateelectrode;

[0040]FIG. 18 is a schematic top view showing an example of dispositionin which a window is provided in a circumferential form symmetricallywith respect to an optical axis of an electron beam optical system;

[0041]FIG. 19 is a schematic top view showing an example of dispositionin which windows are provided symmetrically with respect to an axis ofdeflection direction;

[0042]FIG. 20 is a schematic top view showing another example ofdisposition in which windows are provided in a parallel formsymmetrically with respect to an axis of deflection direction;

[0043]FIG. 21 is a perspective view of a standard calibration patternhaving a slope part;

[0044]FIG. 22 is a schematic section view showing an automaticinspection system in which the standard calibration pattern is securedto an X-Y stage;

[0045]FIG. 23 is a graph for explaining a relationship between objectivelens control current and specimen surface height;

[0046]FIG. 24 is a perspective view of a standard calibration patternhaving two step parts;

[0047]FIG. 25 is a schematic sectional view showing an automaticinspection in which the standard calibration pattern is mounted on a Zstage;

[0048]FIG. 26 shows a relationship between deviation in measurementposition and error in height detection;

[0049] FIGS. 27(a) and (b) show views of a specimen surface forexplaining a method of presuming an observation region height usingheight data detected continuously;

[0050] FIGS. 28(a)-(c) show views of a specimen surface for explaining amethod of presuming an observation region height using height datadetected continuously;

[0051] FIGS. 29(a) and (b) show views of a specimen surface forexplaining a method of presuming an observation region height usingheight data detected continuously in a different manner;

[0052]FIG. 30 is a schematic sectional view of a specimen chamber inwhich a height detection optical system can be moved in parallel to anelectron optical system;

[0053]FIG. 31 is a schematic section view of a specimen for explaining aheight detection error due to nonuniform reflectance on a specimensurface;

[0054]FIG. 32 is a schematic sectional view of an optical system inwhich two slit light beams are projected symmetrically for detection;and

[0055] FIGS. 33(a)-(c) show diagrams for explaining height detectionusing a plurality of fine slit light beams.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0056] Referring now to the accompanying drawings wherein like referencenumerals are utilized to designate like parts throughout the views,there is shown in FIG. 2 an overview of an automatic semiconductordevice inspection system using electron beam images as an exemplarypreferred embodiment of the present invention. In an electron opticalsystem shown in FIG. 2, an electron beam emitted from an electron gun 1is converged through an objective lens 2, and the electron beam thusconverged can be scanned over a surface of a specimen in an arbitrarysequence. A signal of secondary electrons 4 produced on a surface of aspecimen wafer 3 in irradiation with the electron beam is detected by asecondary electron detector 5, and then the secondary electron signal isfed to an image input part 6 as an image signal.

[0057] The specimen wafer under inspection can be moved by an X-Y stage7 and a Z stage 8. By moving each stage, an arbitrary point on thesurface of the specimen wafer is observable through the electron opticalsystem. Electron beam irradiation and image input can be performed insynchronization with stage movement, which is controlled under directionof a control computer 10. A height detector 11 is of an opticalnon-contact type which does not cause interference with the electronoptical system, and it can speedily detect a height of the specimensurface at or around an observation position in the electron opticalsystem by a height calculator 11 a. Resultant data of height detectionis input to the control computer 10.

[0058] According to the height of the specimen surface, the controlcomputer 10 adjusts a focal point of the electron optical system, i.e.,a position of the Z stage, and it receives input of the image signal.Using the image signal input in a focused state and inspection positiondata detected by a position monitoring measurement device, defectjudgment is carried out through comparison with a pattern pre-stored byan image processing circuit 9, a corresponding pattern at a location onthe specimen wafer under inspection, or a corresponding pattern on adifferent wafer with a defect being detected by defect detector 100.While the automatic semiconductor device inspection system usingsecondary electron images is exemplified in FIG. 2, back scatteredelectron images or transmitted electron images may also be used forspecimen surface observation instead of secondary electron images.

[0059] In the example shown in FIG. 2, a spot or slit light beam isprojected onto the specimen surface, reflected light therefrom isimaged, and a position of a light beam image thus attained is detectedfor determining a height of the specimen surface (hereinafter referredto as a light-reflected position detecting method). More specifically,as shown in FIG. 3, the spot or slit light beam is projected onto thespecimen surface at a predetermined angle of incidence so that its imageis formed on the specimen surface, and reflected light thereof from thespecimen surface is detected. Through conversion from specimen surfaceheight variation to light beam image shift, a degree of light beam imageshift is detected to determine a height of the specimen surface.

[0060] The height detector described above may also be applicable todifferent types of microstructure observation/fabrication systems usingother convergent charged particle beams as in the inspection systemexemplified in FIG. 2. The following exemplary preferred embodiments ofthe height detector are described as related to a microstructureobservation system using a charged particle beam, but it is apparentthat the height detector may also be applicable to a microstructurefabrication system using a charged particle beam. As will be apparent tothose skilled in the art, the degradation in image quality in themicrostructure observation system corresponds to the degradation infabrication accuracy in the microstructure fabrication system. It isalso apparent that the present invention is not limited in itsapplication to a charged particle beam system in which a chargedparticle beam is converged to a single point. The present invention isfurther applicable to such microstructure fabrication systems thatimages of an aperture, mask, etc. are formed/projected, and it providessimilar advantageous effects in these systems having image-formingcharged particle optics. As an example of such microstructurefabrication systems, there is an electron beam lithography system usingcell-projection exposure.

[0061] In the light-reflected position detecting method mentioned above,since a height detection optical element is not located directly above adetection position, a height in an observation region in a chargedparticle beam optical system can be detected simultaneously withobservation by the charged particle beam optical system in a fashionthat virtually no interference takes place. By making a height pointdetected by the height detector meet an observation region in thecharged particle beam optical system, a surface height of an object itemcan be known at the time of observation. In this arrangement, throughfeedback of height data thus attained, observation can be conductedusing a charged particle beam which is always in focus.

[0062] It is not necessarily required to provide such a condition that adesired observation region in the charged particle beam optical systemmeets a corresponding height point detected by the height detector, butrather it is just required that a surface height of the object isrecognizable at the time of observation using vicinal height dataattained successively. In use of the light-reflected position detectingmethod, optical parts may be arranged flexibly to some extent in opticalsystem design, and it is therefore possible to dispose the optical partsto prevent interference with the charged particle beam optical system.

[0063] Disposition of the height detector in the light-reflectedposition detecting method is substantially limited by an angle ofincidence on the object surface. In the light-reflected positiondetecting method, since a degree of incidence angle has an effect onheight detection performance, an incidence angle cannot be determinedonly by part disposition in the system. FIG. 4 shows incidence angledependency of surface reflectance of silicon and a resist which arerepresentative materials used in formation of semiconductor wafercircuit patterns. A value of reflectance on specimen surface increaseswith an increase in incidence angle, and a difference in reflectancebetween materials decreases with an increase in incidence angle. Thistendency characteristic also holds for other kinds of materials. Anydifference in reflectance between materials causes nonuniformreflectance on the specimen surface, causing irregularity indistribution of the quantity of light detected. If irregulardistribution of the quantity of light occurs in a detected slit imagedue to nonuniform reflectance of specimen surface pattern, an errortakes place in slit position detection, resulting in a decrease inaccuracy of height detection.

[0064] Referring to FIG. 3, a degree of light beam image shift isdetected by a position sensor. Instead of the position sensor, a linearimage sensor or any sensor capable of detecting a light beam irradiatingposition may also be used. For ensuring a proper S/N ratio in output ofsuch a sensor, it is required to detect an adequate quantity of light.To provide a sufficient quantity of light for stable detection, it isdesirable to increase the incidence angle. In principle, detectionsensitivity in the light-reflected position detecting method becomehigher as the incidence angle with respect to the vertical increases. Anadequate quantity of detected light can be ensured by providing anarrangement that the incidence angle is 60 degrees or more. Moreparticularly, it has been determined that 70 degrees provides goodresults.

[0065] Exemplary preferred embodiments of disposition of optical partsin a height detection optical system are described in the followingdescription wherein in general, if an insulator is located in thevicinity of a charged particle beam optical system, a possible chargebuild-up in the insulator affects an electric field around it to causean adverse effect on charged particle beam deflection, resulting indegradation in image quality. Since such a charging effect varies withtime as a charged condition changes, compensation for it is difficultpractically.

[0066] For attaining a stable charged particle beam image, dispositionof an insulator such as a lens at a position encountered with thecharged particle beam must be avoided. If the insulator is coated with aconductive film and disposed at a position sufficiently apart from thecharged particle beam optical system, an adverse effect may be reduced.A degree of requirement for preventing an adverse effect of theinsulator (lens) on the charged particle beam optical system depends onspecifications of the charged particle beam optical system such asvisual field condition, accuracy, resolution, etc. According to thespecifications of the charged particle beam optical system, a rangeinfluential on the charged particle beam optical system may bedetermined, and an optical path may be designed so that the insulator isnot disposed in the influential range, thus preventing an adverse effecton the charged particle beam optical system.

[0067] When a lens for the height detector is disposed in the peripheryof the charged particle beam optical system, an effect on the chargedparticle beam can be presumed experimentally through computersimulation. The height detection optical system may be designed afterdetermining a suitable mounting position of each lens as illustrated inFIG. 5. A distance between a surface of a specimen (imaging point) andeach of lenses 16 and 17 facing the specimen may be adjusted byselecting lenses having a proper focal length.

[0068] In the preferred embodiment mentioned above, each lens isdisposed at a position which does not cause an adverse effect on thecharged particle beam optical system. Further, as shown in FIG. 6, theremay also be provided such an arrangement that the lenses and other partsof the height detection optical system can be located outside a vacuumspecimen chamber 13 by increasing a distance between the specimensurface and each lens facing the specimen. On a casing between theinside of the vacuum specimen chamber 13 and the atmosphere, there maybe provided a transparent window made of glass or the like. In thisarrangement wherein the optical parts of the height detection opticalsystem are disposed outside the vacuum specimen chamber, adjustment atthe time of installation and maintenance thereafter will be easieradvantageously than when the height detection optical system is disposedin a vacuum as shown in FIG. 7.

[0069] As in the preferred embodiment exemplified above, some or all ofthe optical parts of the height detection optical system may be arrangedoutside the vacuum specimen chamber. As illustrated in FIG. 8, wheresome or all of the optical parts are disposed outside the vacuumspecimen chamber, an external wall for separation between the inside ofthe vacuum specimen chamber and the atmosphere is located on an opticalpath. For allowing passage of light through the external wall, it isnecessary to provide an entrance window made of transparent materialsuch as glass. In an arrangement that the entrance window is formedalong a plane of the external wall at the top of the vacuum specimenchamber as shown in FIG. 8, if a light beam is projected at a high angleof incidence in the light-reflected position detecting method, anincidence angle of the light beam to the entrance window becomes largerto increase reflectance on a surface of the entrance windowsignificantly.

[0070] Referring to FIG. 9, there is shown incidence angle dependency ofsurface reflectance of a representative kind of glass BK7 which iscommonly used as an optical material. Since the surface of the entrancewindow may be coated with a conductive film and different kinds ofwindow materials may be used, the incidence angle dependency will varyto some extent but its tendency characteristic is similar. As theincidence angle to the surface of the entrance window increases, a valueof surface reflectance increases to cause larger loss in the quantity oflight at passage through the entrance window.

[0071] As shown in FIG. 8, light may pass through two windows; anentrance window when it is projected onto a surface of a specimen, andan exit window after it is reflected therefrom. As the number of windowsthrough which light passes is increased, loss in the quantity of lightbecomes larger. Further, in consideration of incidence angledistribution in the light beam (e.g., incidence angle distribution in arange of ±5.7 deg. in case of NA 0.1), it is required to avoid providingan incidence angle which causes significant variation in reflectance inorder to prevent irregular distribution of the quantity of light in thebeam.

[0072] Accordingly, as shown in FIG. 10, there may be provided such anarrangement that an entrance window 23 is formed perpendicularly to orat an angle which is almost perpendicular to the optical path of theheight detection optical system for reducing surface reflectance on thewindow, thereby decreasing loss in the quantity of light on the opticalpath. In consideration of possible irregularity in distribution of thequantity of light in the beam, it is preferred to dispose the entrancewindow at an incidence angle of 30 deg. or less so that there will occurlittle variation in reflectance with incidence angle as indicated inFIG. 9. In addition to the external wall for separation between theinside of the vacuum specimen chamber and the atmosphere, there may beany member part on the optical path in the height detection opticalsystem. If it is impossible to provide an opening through the memberpart, it is required to arrange a window thereon in the same manner. Insuch a case, loss in the quantity of light can be minimized by forming ashape of the window perpendicularly to the optical path as far aspossible on condition that the shape of the window does not cause anadverse effect on the charged particle beam optical system.

[0073] The following description describes exemplary preferredembodiments for reducing an effect of chromatic aberration due tovariance in refractive index of glass material used for a window forlight passage. When a light beam for height detection passes though thewindow made of glass, its optical path is made to shift. As shown inFIG. 11, since there is variance in refractive index of glass material,a degree of optical path shift varies depending on wavelength. Whenwhite light is used for specimen surface height detection, an error mayoccur in height detection due to chromatic aberration caused by thewhite light.

[0074] Further, the degree of optical path shift is dependent on anangle of incidence and proportional to a thickness of glass plate. Ifthe incidence angle to the glass plate of the window is decreased as inthe foregoing preferred embodiment, the degree of optical path shift canbe reduced. However, if the incidence angle is rather large, therearises a particular problem. (For example, in case that the incidenceangle is 70 deg., glass BK7 is used and the thickness of glass plate is2 mm, there occurs a difference of 9 μm in optical path shift betweenwavelengths of 656.28 nm and 404.66 nm.)

[0075] Where white light is used, an effect of chromatic aberrationvaries with color of an object under inspection and therefore itscorrection is rather difficult. For reduction in effect of chromaticaberration, there may be provided such arrangements that the windowglass plate is made thinner and a glass plate for correcting chromaticaberration is inserted on the optical path. Since the degree of opticalpath shift is proportional to the thickness of window glass plate, it ispreferred to use a glass plate having a thickness which will not causesignificant chromatic aberration, in consideration of applicablewavelength coverage and desired accuracy of height detection.

[0076] It is not necessarily required to use glass material if arequired strength can be satisfied, and therefore an opticallytransparent part made of pellicle material, for example, may beemployed. However, in case of the window on the vacuum specimen chamber,considerable strength is required and it is not permitted to make theglass plate sufficiently thinner. Therefore, in such a case, the glassplate for correcting chromatic aberration may be inserted on the opticalpath.

[0077] Referring to FIG. 12, there is shown an arrangement that achromatic aberration correcting glass plate is inserted in the samepositional relation as that of an entrance window with respect to animaging lens. In this arrangement, a difference in degree of opticalpath shift can be canceled by disposing the chromatic aberrationcorrecting glass plate, which has the same characteristic as theentrance glass window in that it, for example, is made of the samematerial as that of the entrance window and has the same thickness asthat of the entrance window, so that an incidence angle to the chromaticaberration correcting glass plate will be θ with respect to an incidenceangle to the entrance glass window θ. A similar arrangement may also beprovided on the detector side with respect to the exit glass window.

[0078] Further, in FIG. 13, there is shown an arrangement that achromatic aberration glass plate and an imaging lens are located inreverse. In this arrangement, a difference in degree of optical pathshift can also be canceled by disposing the chromatic aberrationcorrecting glass plate, which is made of the same material as that ofthe entrance window and has a thickness proportional to a magnificationof the imaging lens, so that the chromatic aberration correcting glassplate will be in parallel to the entrance window.

[0079] For the purpose of decreasing an accelerating voltage for thecharged particle beam to be applied onto a specimen, a flat-plateelectrode may be arranged at a position over a surface of the specimenin parallel thereto. In this arrangement, it is required to provide anopening or window on the flat-plate electrode to allow passage of lighton an optical path for the height detector. Since a shape of theflat-plate electrode has an effect on electric field distribution in thevicinity of the specimen, it may affect the quality of charged particlebeam images adversely. Exemplary embodiments for reducing an adverseeffect on the charged particle beam images are described in thefollowing description. A degree of adverse effect on the chargedparticle beam optical system varies depending on the size or position ofthe opening to be provided on the flat-plate electrode. An permissiblelevel of adverse effect by the opening depends on performance requiredfor the charged particle beam optical system. When the size of theopening is considerably small, its adverse effect may be negligible.Therefore, a method for reducing the opening size is explained below.

[0080] As shown in FIGS. 14(a) and 14(b), when an incidence angle to asurface of an object with respect to the vertical is increased from thesmall incidence angle of FIG. 14(a) to the relatively large incidenceangle of FIG. 14(b), the size of an optical path going through a planeparallel to the object surface becomes larger even if a numericalaperture (NA) of the optical path of the height detection optical systemis constant. Where the optical path goes through an opening on theflat-plate electrode 25 as in this case, the shape of the opening 26must be enlarged substantially in the projecting direction of theoptical axis to the flat-plate electrode from that shown in FIG. 14(a)to that shown in FIG. 14(b). This gives rise to a problem particularlyin a situation where the numerical aperture of the optical system israther large and a distance between the flat-plate electrode and theobject surface is rather long. A suitable position of the flat-plateelectrode is determined according to specifications of the chargedparticle beam optical system, and it cannot be changed in commonapplications. Further, it is not allowed to extremely decrease thenumerical aperture since a sufficient quantity of light must be providedfor detection.

[0081] Reduction of the size of the opening without decreasing theentire quantity of light for detection is described below. Commonly, anoptical lens aperture having a circular shape whose center coincideswith the optical axis is employed. According to one aspect of thepresent invention, there is provided an elliptic or rectangular opticallens aperture having its major axis which is in the axial directionacross the optical axis and parallel to the object surface and havingits minor axis which is in the axial direction across the major axis andthe optical axis. In this arrangement, the entire quantity of lightnecessary for height detection can be ensured by providing an ellipticor rectangular area which is equal to that of a circular lens aperture.

[0082]FIG. 15 shows an optical geometry of an optical path going throughthe opening 26 of the flat-plate electrode 25 in case of a circularoptical aperture, and FIG. 16 shows an optical geometry of an opticalpath going through the opening 26 of the flat-plate electrode 25 in caseof an elliptical optical aperture which has almost the same area as thatof the circular optical aperture in FIG. 15. As can be seen from thesefigures, the size of the opening 26 in one direction on the flat-plateelectrode 25 can be reduced by using the elliptic aperture. Asillustrated here, the size and shape of the opening can be changed bymodifying the shape of the aperture as far as performance required forthe height detector can be ensured. Thus, a degree of adverse effect onthe charged particle beam optical system can be reduced.

[0083] If the charged particle beam optical system is affected by thesize of the opening so that performance required for it cannot beattained, it is necessary to provide a further measure. For example,instead of merely a hollow opening formed on the flat-plate electrode,there may be provided such an arrangement that a window made of glasscoated with a conductive film or other material is formed on theflat-plate electrode to allow passage of light on an optical path. Inthis arrangement, an adverse effect due to electric field to be given toan object or its periphery can be reduced. As exemplified in FIG. 8, ifthe window is formed at the position of the opening along a plane of theflat-plate electrode in FIG. 14, significant loss in the quantity oflight occurs due to reflection on a surface of the window, causingirregular distribution in the quantity of light in the beam. Therefore,as exemplified in FIG. 10, there may be provided such an arrangementthat the window is formed perpendicularly to or at an angle almostperpendicular to the optical path. Thus, loss in the quantity of lightdue to reflection on the surface of the window can be decreased. FIG. 17shows an example of the window formed in this arrangement.

[0084] The opening or window formed on the flat-plate electrode in theforegoing examples has a considerable effect on electric potentialdistribution in the vicinity of the object. The following describes anopening/window disposition method for reducing this effect. Since thewindow and opening can be disposed in the same manner, the window istaken in the description given below.

[0085] In a microstructure observation/fabrication system to which thepresent invention is directed, two-dimensional observation orfabrication is mostly carried out through two-dimensional scanning bydeflecting a convergent charged particle beam or through stage scanningby combination of one-dimensional scanning based on charged particlebeam deflection and stage movement in the direction orthogonal to theone-dimensional scanning. According to the present invention, the windowis disposed in consideration of charged particle beam deflection andstage movement direction in charged particle beam scanning. Thus, aneffect of variation in electric field due to the window can be reducedas proposed below.

[0086] Referring to FIG. 18, there is shown an example of disposition inwhich the window 29 is provided in a circumferential form having itscenter at the optical axis of the charged particle beam optical system.Since the window is located at a position apart from a scanning range ofthe charged particle beam, an effect of variation in electric field dueto the window is isotropic in the disposition shown in FIG. 18. Thus,the effect will be almost uniform in an observation region in thecharged particle beam optical system. Further, it is possible to attainalmost the same result by disposing dummy windows 30 at axisymmetricpositions with respect to the directions of electron beam deflection andstage movement as shown in FIG. 19.

[0087] In case of stage scanning, electric field distribution in adeflection range can be made uniform by disposing windows 29 in parallelto the deflection direction as shown in FIG. 20. If electric fielddistribution is kept uniform, scanning position correction is allowed toenable improvement in image quality. In carrying out the presentinvention, an effect to be given by the shape and disposition of thesewindows or openings is to be examined in consideration of specificationsof the charged particle beam optical system and desired inspectionperformance to select suitable window formation and disposition.

[0088] The following describes exemplary embodiments for chargedparticle beam focus adjustment using height detection result dataattained by the height detector. A focal point of the charged particlebeam is adjusted by an objective lens control current. Using input dataof an object surface height detected by the height detector in anobservation region of the charged particle beam optical system, theobjective lens control current is regulated to enable observation of acharged particle beam image which is always in focus. For this purpose,in the charged particle beam optical system, a level of objective lenscontrol current is to be calibrated beforehand with respect to variationin object surface height. Further, an offset and gain in relationbetween the height detector and the charged particle beam optical systemare to be calibrated beforehand.

[0089] Calibration methods for offset and gain will be described in thefollowing exemplary embodiments. When the charged particle beam opticalsystem is not structured in a telecentric optical arrangement, variationin object surface height will cause a magnification error in addition toa defocused condition. As to the magnification error, correction can bemade through feedback control of a deflection circuit using heightvariation data, thus making it possible to always attain a chargedparticle beam image at the same magnification. Further, if themicrostructure observation/fabrication system using the convergentcharged particle beam is provided with a mechanism capable of moving anobject in the Z-axis direction with high accuracy and at response speedsufficient for focal point control, resultant data of height detectionmay be used for object stage height feedback control instead of feedbackcontrol of the charged particle beam optical system.

[0090] Where stage height feedback control is carried out, a surface ofthe object can always be maintained at a constant height with respect tothe height detector and the charged particle beam optical system.Therefore, no problem will arise even if a guaranteed detection accuracyrange of the height detector is narrow. As a drive mechanism for anobject stage, there may be provided a piezoelectric mechanism enablingfine movement at high speed under vacuum, for example. When such apiezoelectric mechanism is used, a magnification error does not occursince a height of the object surface is always maintained at a constantlevel with respect to the charged particle beam optical system.

[0091] Calibration of objective lens control current and focal point inthe charged particle beam optical system may be carried out in thefollowing manner. In an instance where there is a nonlinear relationshipbetween objective lens control current and focal point, it is requiredto make correction for nonlinearity. Linearity evaluation and correctionvalue determination may be effected as described below.

[0092] Referring to FIG. 21, there is shown a standard pattern 31 a forcalibration. As shown in FIG. 22, this standard calibration pattern issecured to a stage for holding an object. The standard calibrationpattern is made of conductive material so that it will not be charged byscanning of the charged particle beam. It is also desirable to providesuch a surface pattern feature that a height at each position can beidentified.

[0093] When the object holding stage is movable on a plane as in theinspection system shown in FIG. 2, the standard pattern is moved to anobservation region at the time of calibration. Using the standardpattern, objective lens control current measurement is effected todetermine a current level where a charged particle beam image becomessharpest at each point. At this step, visibility of the charged particlebeam image is determined through visual observation or image processing.In this measurement, it is possible to determine a relationship betweenvariation in object surface height and optimum level of objective lenscontrol current as shown in FIG. 23. If the relationship betweenvariation in object surface height and optimum level of objective lenscontrol current is determined, a value of objective lens control currentwhich is most suitable for forming the charged particle beam image infocus can be identified using object surface height data attained by theheight detector.

[0094] The standard pattern 31 a shown in FIG. 21 has a flat part atboth ends thereof. At each flat part, if a reference height isdetermined through measurement with the optical height detector,gain/offset calibration of objective lens control current can be madeaccording to height measurement data. In case that characteristics ofobjective lens control current and focal point are calibrated for theobjective lens by any means, gain/offset calibration of objective lenscontrol current may be made with respect to the optical height detectorusing a standard pattern 31 b which has two step parts as shown in FIG.24.

[0095] Where the object holding stage is not provided with a movementmechanism, the charged particle beam optical system can be calibrated bydisposing the standard pattern so that it will always be located in avisual field of the charged particle beam optical system. Further, thestandard pattern may be formed so that it can be attached to an objectholding jig. Thus, even when the object holding stage is not providedwith a movement mechanism, it is possible to perform calibration bysetting the standard pattern on the stage and thereafter exchange thestandard pattern with the object for observation.

[0096] In case that the charged particle beam system is provided with amechanism for moving an object in the height direction as shown in FIG.25, an ordinary stepless pattern is utilizable instead of the standardpattern shown in FIG. 21. Through height detection by Z stage movementand image evaluation using the stepless pattern, calibration ofobjective lens control current can be made with respect to the heightdetector. Where there is provided a movement mechanism for Z stage, itis possible to conduct focus adjustment using the Z stage. However, if aresponse speed of the Z stage is not sufficiently high for anobservation region change speed, focal adjustment may be made using theobjective lens control current with the stage being fixed.

[0097] Calibration of the charged particle beam optical system using thestandard pattern shown in FIG. 21 is practicable only in amicrostructure observation/inspection system which allows observation ofa surface feature of the standard pattern using the charged particlebeam optical system. As contrasted, in a microstructure fabricationsystem, calibration is to be made only for the height detector using thestandard step-pattern shown in FIG. 24, and for a relationship betweenfocal point and control current of the charged particle beam opticalsystem, calibration is made beforehand therein. Where the microstructurefabrication system is provided with a charged particle beam imageobservation mode in which such an operational parameter as anaccelerating voltage for the convergent charged particle beam can bealtered, it is possible to check a point detected by the height detectorusing a charged particle beam image.

[0098] The following describes exemplary embodiments concerning focalpoint correction and relationship between height measurement positionunder inspection and observation position in the charged particle beamoptical system. If the observation position of the charged particle beamoptical system completely meets the height detection position of theheight detector, focus adjustment may be made according to height datadetected by the height detector. However, in the light-reflectedposition detecting method, a deviation of detection position occurs dueto variation in object surface height as illustrated in FIG. 3.Designating a predictable value of maximum variation in object surfaceheight as Zmax and an incidence angle in the height detection opticalsystem as θ, a value of maximum positional deviation Xmax is equal toZmax.tanφ. Then, on condition that a value of allowable variation inobject surface height in terms of focal depth of the charged particlebeam optical system and performance requirement for the system is z0 anda predictable value of maximum gradient of object surface is Δmax, avalue of height detection error for maximum positional deviation dz isexpressed as Δmax·Xmax=Δmax·Zmax·tanΘ as indicated in FIG. 26. If theheight detection error dz is smaller than z0, there arises no problem.However, if dz is larger than z0, it is required to attain a height onthe optical axis of the charged particle beam optical system.

[0099] In the inspection system according to the present invention,since continuous inspection is performed by moving the stage, heightdata at each point can be attained continuously. Using resultant data ofheight detection, a height of object surface in an observation region inthe charged particle beam optical system may be presumed or predicted toenable focus adjustment. Focus adjustment when there is a positionaldeviation between the height detection position and the observationregion in the charged particle beam optical system may be effected inthe following manner. In the following description, it is assumed thatstage scanning is performed by deflecting the beam of the chargedparticle beam optical system in the Y-axis direction and moving thestage in the X-axis direction to produce a two-dimensional image.

[0100] Where each of X-axis and Y-axis stage scanning movements isalways limited to one direction at the time of inspection, if each ofthe X-axis and Y-axis scanning movements is always made in one directiononly as shown in FIG. 27, i.e., reciprocal scanning movement is notperformed, the height detector may be disposed with an offset so thatthe height detection position will always be located before theobservation position of the charged particle beam optical system withrespect to the direction of stage scanning movement as shown in FIG.27(a). In this manner, a height at a desired position can be determinedusing height data in the vicinity of the observation region, which isattainable before each step of inspection.

[0101] As shown in FIG. 27(b), three points in the vicinity of thecurrent inspection position are selected and a height of the inspectionposition is presumed according to a local plane determined by thesethree points. It is necessary to select three points so that the currentinspection position will be located inside a triangle formed with theselected three points. Thus, a height of the inspection position can bepresumed reliably through interpolation. In this case, although a heightof a stage scanning position at the start of inspection cannot bepresumed, it can be determined by performing a sequence of scanning forheight detection in advance.

[0102] Another exemplary embodiment is considered in that either one ofX-axis and Y-axis stage scanning movements is always limited to onedirection and also the axis movable only in one direction coincides withthe projection direction of the height detection optical system. Asshown in FIG. 28, if the X-axis stage scanning movement is alwayslimited to one direction and the X axis coincides with the projectiondirection of the height detection optical system, positional deviationin height detection due to variation in height takes place only in theX-axis direction. Therefore, by providing an offset in the X-axisdirection as shown in FIG. 28(a), a height can be determined throughone-dimensional interpolation using height data on one line only. Inthis case, a height of the inspection position may be determined bymeans of linear interpolation using two-point data or splineinterpolation using three-point data. At the start of inspection, aheight detection value in an entrance section until the stage reaches aconstant speed may be used.

[0103] Further, as shown in FIG. 29, if the Y-axis stage scanningmovement is always limited to one direction and the Y axis correspondsto the projection direction of the height detection optical system,positional deviation in height detection due to variation in heighttakes place only in the Y-axis direction. Therefore, by providing anoffset in the Y-axis direction as shown in FIG. 29(a), a height of theinspection position can always be determined reliably throughinterpolation using height detection data on a preceding line. In casethat the stage is moved in a reciprocal scanning fashion, such an offsetas mentioned above cannot be provided in one direction.

[0104] In an arrangement that the optical axis of the charged particlebeam optical system is made to coincide with a reference position ofheight detection, it is possible to presume a height of the inspectionposition using height detection data attained. However, since a heightof the inspection position cannot always be determined throughinterpolation, its reliability is not ensured. For reliable heightdetection, there may be provided such an arrangement that the heightdetection optical system is equipped with a movable mechanism and theentire optical system is shifted in parallel as shown in FIG. 30 so asto give an offset in the stage scanning movement direction. Thus, aheight of the inspection position can always be determined reliablythrough interpolation in the same manner as in the foregoing example.There may also be provided such an arrangement that a plurality ofheight detectors are disposed to enable height measurement at aplurality of points in the vicinity of the inspection position. In thisarrangement, data of only necessary points can be used according to thestage scanning movement direction.

[0105] Exemplary embodiments for optical height detection in which aheight of a specimen surface can be detected reliably without beingaffected by a state of the specimen surface are now considered. In casethat a specimen surface height is detected by the light-reflectedposition detecting method as shown in FIG. 3, a deviation of a detectionposition occurs to cause an error in height detection. As shown in FIG.31, if a specimen surface 32 is provided with pattern areas havingdifferent reflectances (high reflectance area 36, low reflectance area37) and slit light is projected onto a pattern boundary 38 therebetween,reflected light intensity distribution 34 of slit light to be detectedis affected to cause an error in height detection. Such a heightdetection error may be reduced in the following manner. As shown in FIG.32, two slit light beams are projected onto the specimen surface indirections symmetrical with respect to a normal line thereon, andrespective reflected light beams from the specimen surface are detected.If sensors for detecting these slit light beams are disposed as shown inFIG. 32, a light image shift due to variation in specimen surface heightis made in the same direction and a measurement error due to specimensurface features appears in the opposite directions. Therefore, aneffect of specimen surface pattern features can be canceled by means ofaddition. Further, in case that the slit light beams are projected intwo directions as shown in FIG. 32, a deviation of the detectionposition due to variation in height occurs to the same extent in theopposite directions. Therefore, a deviation of the detection positioncan be eliminated by means of averaging.

[0106]FIG. 33 shows a method for reducing an effect of specimen surfacepattern features using a plurality of fine slits. A height detectionerror due to specimen surface pattern features increases in proportionto a slit width. Therefore, as shown in FIG. 33(a), a plurality of fineslit light beams are projected onto the specimen surface, and reflectedlight beams are detected by a linear image sensor. Individual centervalues of plural slit beam images are determined and averaged, thusmaking it possible to reduce an error in height detection. As shown inFIG. 33(c) in comparison with FIG. 33(b), an error on a pattern boundarycan be reduced by decreasing each slit width. Since fine slit beams onother than the pattern boundary are not affected by pattern features, anerror on the pattern boundary can be decreased through averaging.Although the quantity of light to be detected decreases as each slitwidth is decreased, an S/N ratio can be improved by averaging for pluralslit positions, thereby ensuring reliability in height detection.

[0107] According to the present invention, it is possible to detect aheight of an observation position in the electron beam optical systemusing the optical height detector and attain an in-focus electron beamimage while conducting inspection. In an electron beam inspectionsystem, inspection performance and reliability thereof can be improvedby carrying out inspection using an electron beam image which is alwaysfocused in a consistent state. Furthermore, since height detection canbe made simultaneously with inspection, continuous stage movement isapplicable to inspection to reduce a required inspection timesubstantially. This feature is particularly advantageous in inspectionof semiconductor wafers which will become still larger in diameter inthe future. Similarly, the same advantageous effects can be attained ina microstructure observation/fabrication system using a convergentcharged particle beam. Further, by disposing the height detectionoptical system outside the vacuum specimen chamber, adjustment andmaintenance can be carried out with ease.

[0108] While we have shown and described several embodiments inaccordance with the present invention, it is understood that the same isnot limited thereto but is susceptible of numerous changes andmodifications as known to those skilled in the art, and we therefore donot wish to be limited to the details shown and described herein butintend to cover all such changes and modifications as are encompassed bythe scope of the appended claims.

What is claimed is:
 1. A convergent charged particle beam apparatuscomprising: an electron beam system which emits a converged electronbeam; a vacuum chamber which is connected to the electron beam system; astage which mounts a specimen and moves at least in one direction insideof the vacuum chamber; an electron beam image observation unit whichobserves an electron beam image of a surface of the specimen mounted onthe stage by irradiation of the electron beam emitted from the electronbeam system and scanned over the surface of the specimen; a heightdetector which optically detects a height of the specimen mounted on thestage by illuminating the surface of said specimen with light and bydetecting reflected light of the illumination reflected from the surfaceof the specimen; and a controller which controls a focus position of theelectron beam in accordance with an output from the height detectorwhile the stage is moving at least in one direction.